Patient beds in healthcare facilities are designed so that various parts of the bed can adopt a number of positions to provide for greater patient comfort and/or to facilitate the tasks of an attendant, for example a nurse. For example, beds may be raised or lowered to different heights. Beds may be tilted to achieve the Trendelenburg and reverse Trendelenburg positions. Beds may comprise patient support platforms having back rests and/or knee rests that can be raised or lowered to support a patient's back and knees in a variety of positions.
Adjusting the height of a patient bed may be accomplished by a variety of means. One particularly advantageous method is through the use of linear actuators, for example as described in U.S. Patent Publication 2003/0172459 published Sep. 18, 2003, the disclosure of which is herein incorporated by reference. In such a bed, the head end and the foot end of the bed are raised or lowered through the use of separate linear actuators. One linear actuator operates a first set of pivotable legs for adjusting the height of the head end of the bed while another linear actuator operates a second set of pivotable legs for adjusting the height of the foot end of the bed. However, since the two linear actuators operate separately, there is a tendency for one end of the bed to lag behind the other, thereby causing the bed to acquire a tilt. This problem is exacerbated when there is unequal loading on one end as opposed to the other end of the bed since the linear actuator at the end with greater loading must work harder to adjust the height of that end.
A number of methods have been used to mitigate against this problem. For example, limit switches or stops may be used on the bed to deactivate the lead linear actuator at pre-set intervals to provide time for the other to catch up. However, the necessarily wide spacing of such limit switches still results in significant and noticeable tilting of the bed between intervals. As well, motion of the bed during height adjustment is noticeably fitful and uneven.
U.S. Pat. No. 5,205,004 issued Apr. 27, 1993 to Hayes et al. describes the use of an external level sensor connected to actuators so that if the tilt of the bed varies from the adjusted and desired position, one or the other actuator is adjusted to restore the desired tilt position. This system has several drawbacks. Since the sensor is located externally from the actuators, it can get in the way of normal bed operation and may be subject to physical damage. Furthermore, external sensors described in this patent lack sensitivity and lead to noticeable tilt and fitfulness during height adjustment of the bed.
Finally, it has even been suggested in the art to use very powerful linear actuators, which are not affected by the load on the bed. However, this has proven to be practically not possible as all actuators have load restrictions. In any event, such very powerful actuators would be overly expensive and would have larger power requirements.
There is still a need in the art for a simple, reliable system for leveling a bed with little noticeable tilt and greater smoothness of operation during height adjustment of the bed.